The present invention relates generally to an image region segmentation system, and more particularly to an image region segmentation system for automatically discriminating a text region from an image in which a line image and a dot image (which includes a continuous-tone dot image and/or a screened halftone dot image) coexist. This system is applicable to digital copying machines and facsimile machines.
In copying machines or facsimile machines, an image which is copied, transmitted or received may be a composite image in which a dot image and a line image coexists. The dot image refers to a photograph, a picture or the like which is described by dots, while the line image refers to a character, a text or the like which is described by lines. In order to improve the quality of the copied, transmitted or received image, it is desirable to carry out a pseudo-halftone generating process with respect to the continuous-tone dot region such as a photograph, to carry out a process of eliminating the moire with respect to the screened halftone dot region, and to carry out a sharpening process with respect to the line region such as a character. In addition, when transmitting the composite image, it is desirable from the point of view of improving the compression rate that a coding process is carried out after processes appropriate for characteristics of various regions of the image are carried out.
In order for carrying out the processes appropriate for characteristics of the various regions of the image, it is necessary to accurtely discriminate between a text region and a picture region within an image as a preliminary process before the above processes are carried out. Conventionally, there is a block adaptive thresholding method which has been used as a method for discriminating a line region from an original image, and this method is hereinafter referred to as the BAT method. This BAT method is disclosed, for example, in "Bilevel Rendition Method for Documents Including Gray-Scale and Bilevel Image" by N. Tetsutani et al. in an article from the the Institute of Electronics and Communication Engineers of Japan (IECEJ), (1984), Vol.J67-B, No.7, pp. 781-788. When the BAT method is applied to an image, the image is divided into a plurality of blocks each having a prescribed size, and the maximum intensity (or the maximum optical density) level and the minimum intensity (or the minimum optical density) level are detected for each of the blocks. If a block among the blocks has a difference between the maximum and minimum intensity levels that is greater than a predetermined threshold value, then it is judged that the block is a line region or a text region within the image. If a block has a difference between the maximum and the minimum intensity levels that is not greater than the threshold value, then it is judged that the block is a gray-scale image region or a picture region which is different from the text region within the image.
However, when the above described BAT method is applied, a screened halftone dot image cannot be discriminated from the original image, and there is a possibility that such a screened halftone dot image is erroneously judged as being a text region of the image. Therefore, in order to accurately discriminate a text region within the image when the BAT method is applied, it is necessary to carry out simultaneously an additional process of discriminating between a screened halftone dot image and a non-screened halftone dot image within the image. Thus, the composite BAT method in such a case becomes a complicated process, and the apparatus for accurately discriminating between a text region and a picture region within an image must have a relatively great, complicated hardware, when compared with the hardware of an apparatus for performing the simple BAT method only.